The present invention relates to a measurement apparatus, and more particularly to measurement apparatus for converting a physical quantity into a digital quantity and for carrying out predetermined processing operations on the digital quantity to obtain measurement data suitable for transmission over an appropriate data path to a central location.
In the measurement of various physical quantities, such as the volume flow rate of a fluid, temperature, pressure, etc., transducers are frequently used to convert the physical quantity to a frequency, voltage, electrical resistance, or electrical capacitance. The output of the transceiver then undergoes further processing, either in its unaltered form or after an appropriate conversion, to produce measurement data.
One known type of transducer for converting a physical quantity into a frequency is a vortex-shedding flowmeter utilizing a von Karman Vortex stream pattern. FIG. 1 is illustrates the princliples of operation of such flowmeter. The flowmeter is depicted in a schematic plan view showing a column-shaped vortex-shedding body OB placed in a fluid flow FL. Vortices BU are generated downstream of the body OB, alternately one on each side thereof, to produce an alternating pressure difference on the two sides of the body such that the pressure on the side where a vortex is formed is lower than that on the side where no vortex is formed. The alternating pressure difference is used to excite a vibratory plate (not shown) in a chamber (not shown) located either in the vortex-shedding body OB or outside thereof. The chamber is in communication with the fluid through openings or pressure transmission ports (not shown). The frequency F of generation of vorticles can be expressed as: EQU F=k.multidot.V/D (1)
where V is the speed of flow of the fluid, D is the diameter of the vortex-shedding body, and k is a constant. The speed of flow or the volume flow rate can be derived from the frequency F using equation (1). In a typical application the frequency signal provided by the flowmeter is converted into a corresponding voltage signal by a frequency-to-voltage (F/V) converter. The voltage signal is then converted into a corresponding current signal suitable for transmission along a conductor. In the prior art apparatus for carrying out such a measurement application use analog circuits for the processing of the measurement signals. However, such prior art measurement apparatus have the problem in that the analog circuits used therein are subject to thermal drift and sensitivity to noise which can reduce the accuracy of the measurement data provided by the apparatus.
Thermocouples or thermistors are widely used as transducers for converting a temperature into a corresponding voltage or resistance, respectively. Generally an operational amplifier is used to amplify the voltage generated by the thermocouple or the voltage drop accross the thermistor and to convert such voltage to a corresponding resistance at the output of the operational amplifier. In the prior art, the processing of the resistance signal to produce temperature measurement data is typically carried out in a measurement apparatus which uses analog circuitry. Consequently, such prior art measurement apparatus have the above described problems associated with the use of analog circuitry for processing of measurement signals. In addition, thermocouples and thermistors have nonlinear temperature vs. voltage and temperature vs. resistance characteristics, respectively. Therefore, it would be desirable for the measurement apparatus to have provisions for linearizing such nonlinear characteristics and for making other corrections and adjustments. Such provisions are difficult to implement with analog circuitry.
Transducers for converting a physical quantity into a corresponding capacitance are also well known. An example of such transducers is the capacitance manometer for measuring absolute or differential pressure. A typical capacitance manometer includes a flexible diaphragm which can be displaced by a differential pressure on the two sides thereof, a movable electrode attached to the diaphragm, and two fixed electrodes positioned one on each side of the diaphragm. The principles of operation of such a transducer will now be described with reference to FIGS. 2(A) and 2(B). In FIG. 2(A), a movable electrode EL.sub.V is disposed between two fixed electrodes both designated EL.sub.F, the movable electrode EL.sub.V being attached to a flexible diaphragm (not shown) and laterally movable in the directions of the arrow R in response to a change in differential pressure on the two sides of the diaphragm. When the movable electrode EL.sub.V is thus displaced, one of the two capacitances CA.sub.1 and CA.sub.2 between the movable electrode and each of the fixed electrodes is increased while the other is decreased by a corresponding amount. Thus, the capacitances are differentially varied. Assuming that the area of each electrode is S, the dielectric constant between the electrodes is .epsilon. and the undisplaced distance between the movable electrode EL.sub. V and each of the fixed electrodes EL.sub.F is d, the capacitances CA.sub.1 and CA.sub.2 when the movable electrode EL.sub.V is displaced .DELTA.d from its undisplaced position, as shown by the dotted line in FIG. 2(A), can be given as follows: EQU CA.sub.1 =.epsilon.A/(d-.DELTA.d), and (2) EQU CA.sub.2 =.epsilon.A/(d=.DELTA.d). (3)
The sum and difference of the capacitances are given by the following equations:
CA.sub.1 +CA.sub.2 =.epsilon.A.multidot.2d/(d.sup.2 -(.DELTA.d).sup.2), and (4) EQU CA.sub.1 +CA.sub.2 =.epsilon.A.multidot.2.DELTA.d/(d.sup.2 -(.DELTA.d).sup.2). (5)
The ratio between the sum and the difference may be expressed by: EQU (CA.sub.1 -CA.sub.2)/(CA.sub.1 +CA.sub.2)=.DELTA.d/d. (6)
Thus, the displacement .DELTA.d can be derived from the capacitance ratio (CA.sub.1 -CA.sub.2)/(CA.sub.1 +CA.sub.2).
In FIG. 2(B) another pressure-to-capacitance transducer structure is shown in which the two fixed electrodes EL.sub.F are disposed adjacent to each other and the movable electrode EL.sub.V is positioned adjacent to one of the fixed electrodes EL.sub.F. When the movable electrode EL.sub.V is displaced .DELTA.d from its undisplaced position, as shown by the dotted line in FIG. 2(B), the capacitances CA.sub.1 and CA.sub.2 can be expressed as: EQU CA.sub.1 =.epsilon.A/d, and (7) EQU CA.sub.2 =.epsilon.A/(d-.DELTA.d). (8)
The difference between the two capacitances is given by: EQU CA.sub.1 -CA.sub.2 =.epsilon.A.multidot..DELTA.d/d(d+.DELTA.d). (9)
The ratio of CA.sub.1 -CA.sub.2 to CA.sub.2 is given by: EQU (CA.sub.1 -CA.sub.2)/CA.sub.2 =.DELTA.d/d. (10)
Thus, the displacement .DELTA.d can be detected as a change in the capacitances.
In a typical application, a capacitance is measured by applying a high-frequency a-c voltage to the capacitance to be measured and deriving the value of the capacitance from the current flowing through the capacitance as a result of the a-c voltage. Prior art measurement apparatus for carrying out the processing of the current signal to derive the capacitance measurement data typically rely on analog circuitry for such processing. Therefore, such apparatus have the above-described problems associated with the use of analog circuitry. Furthermore, the capacitance measurement technique just described is also subject to errors caused by variations in the frequency and magnitude of the a-c voltage. Therefore, a need exists for a measurement apparatus which overcomes the above-described problems of the prior art and which provides measurement data suitable for transmission over an appropriate data path, such as an optical data link, whereby the measurement apparatus may be used in a high reliability measurement system having a plurality of remote measurement stations all coupled to a central processor.